The manufacturing of integrated circuits includes repeatedly projecting a pattern in a lithographic step onto a semiconductor wafer and processing the wafer to transfer the pattern into a layer deposited on the wafer surface or into the substrate of the wafer. This processing includes depositing a resist film layer on the surface of the semiconductor substrate, projecting the pattern onto the resist film layer, and developing or etching the resist film layer to create a resist structure. The resist structure is transferred into a layer deposited on the wafer surface or into the substrate in an etching step. Planarization and other intermediate processes may further be necessary to prepare a projection of a successive mask level.
The pattern being projected is provided on a photo mask. The photo mask is illuminated by a light source having a wavelength, which is selected in a range from visible light to deep-UV in modern applications. The part of the light, which is not blocked or attenuated by the photo mask, is projected onto the resist film layer on the surface of the semiconductor wafer.
In order to manufacture patterns having line widths in the range of 70 nm or smaller, large efforts have to be undertaken to guarantee sufficient dimensional accuracy of patterns projected onto the resist film layer. The dimensional accuracy of patterns depends on many factors, e.g., the illumination condition of the exposure tool, the characteristics of the resist film layer with respect to exposure dose in different regions on the wafer, and under varying illumination conditions. Control of dimensional accuracy is performed by measuring the size of portions of a test pattern of the current layer with an inspection tool. Typically, CD-SEM structures are used to quantify the amount of deviation from the design value, e.g., by using a SEM-tool. It is also quite common to use scatterometry marks, which are inspected with a spectroscopic ellipsometer.
One possibility of assessing the accuracy of critical dimensions is related to focus parameters of the exposure tool used for projecting the pattern. Usually a process window is defined, which allows for small variations of the parameters for illumination. A critical parameter is the depth-of-focus, which gives the amount of allowable focus change. A change in focus usually results in loss of contrast, degrading the resolution of the image projected on the wafer. In order to understand how this translates into critical dimension inaccuracies the response of the resist film layer has to be taken into account. In conventional broad band illumination, i.e., g-line or i-line Mercury light sources with 356 nm wavelength, the observed change in dimensions and edge contrast could be used.
With the advent of light sources having a shorter wavelength, i.e., 248 nm or 193 nm as used nowadays, the determination of line end shortening or reduced edge contrast gets increasingly difficult. Recently chemically amplified resists are available. These type of resists show a very steep characteristic in blackening the resist for a given range of exposure doses. Using such a resist decreases the response of the resist to variations of image contrast. Therefore, conventional methods, as discussed above, are less sensitive to variations of focus, increasing the difficultly in determining the optimum focus settings of the exposure tool.
However, with decreasing feature sizes of patterns the precise determination of dimensional accuracy gets even more important, as those technologies already start with a relatively small process window. Non-systematic errors, like focus parameters, become more and more important. Failing to control those parameters would ultimately result in a low yield of the produced circuits.